Ethical concerns about last week's CRISPR breakthrough in China are valid. But they can evolve quickly.

NEW YORK — It’s too soon to know whether a Chinese researcher who claims to have successfully edited the genomes of newly born twins is telling the truth. But if he is, and if the girls turn out to be healthy and normal, it heralds a significant change in the scientific and ethical status of human gene editing. The outrage might not last long.

The consensus in the scientific community now is that human gene editing is medically dangerous and ethically wrong. Both of those beliefs are susceptible to changing, almost as fast as science is capable of progressing.

And, interestingly, the two main concerns about gene editing using the CRISPR-Cas 9 system are almost diametrically opposed, logically speaking.

The first worry is that CRISPR technology, while cheap and powerful, isn’t reliable enough for use in humans. Specifically, the concern is that changes in one gene that has been “knocked out” and replaced with another could have unforeseen and harmful effects elsewhere in the genome.

These scientific and ethical concerns are serious.

Called “off-target” effects, such unintended modifications are rare but not unheard of in CRISPR experiments in mice. When it comes to humans, the worry goes, an intended mutation that is beneficial or even lifesaving could turn out to have serious, permanent negative effects elsewhere.

The second worry is that CRISPR editing of the human genome will work all too well. That could lead to parents seeking to have designer babies, offspring whose genomes have been edited so that they will be more athletic or more attractive or more intelligent.

The ethical concerns there range from the obvious (it seems too much like eugenics) to the more subtle (it could enhance class differences between those with access to the technology and those without).

Then there’s the fact that gene editing isn’t restricted to that one child. CRISPR genetic mutations are passed on to the next generation, whether for good or for ill.

These scientific and ethical concerns are serious. But they can change fast.

Consider the off-target effects. This worry is based on empirical science: either there is a meaningful probability of dangerous off-target mutations, or there isn’t. If children like the twins who have been reportedly modified are born and live healthy, normal lives, then scientific worries about off-target effects will begin to recede.

As an editorial in Nature Medicine pointed out last summer, all acts of sexual reproduction carry a background probability of spontaneous mutation — that’s why evolution is possible. If the rate of off-target effects is lower than that of natural mutation, scientists and regulators may come to consider it to be tolerable.

That leaves the designer-babies worry — and there are a couple of reasons to predict that it, too, may fade.

Designer babies aren’t a very realistic option.

To begin with, designing taller or smarter babies is not a realistic possibility in the foreseeable future. Most observable human features are associated with hundreds of genetic mutations, not just one or two. One leading study on height found that 697 genetic variants accounted for one-fifth of the difference among people. It isn’t realistic to use CRISPR to knock out and replace those 697 genes to achieve a possible 20 percent gain in height.

Other examples of human variability, such as intelligence, would be even harder to change with current editing techniques. We can’t even produce a consistent definition of intelligence, much less identify its genetic determinants.

As the public gradually realizes that designer babies aren’t a very realistic option, the ethical worry about producing them is likely to fade.

What will remain is the strong ethical value of protecting future generations from debilitating disease. The reported Chinese human editing case was unnecessary, because there are other, simpler ways of protecting a fetus from contracting HIV from a paternal donor. But plenty of other diseases can be avoided only by genetic mutation.

If and when it becomes scientifically safe to proceed with human gene editing, the legitimate ethical concern about designer babies is likely to be outweighed by the ethical imperative to avoid disease, and to enable parents to reproduce who might otherwise not have been able to do so.

Over time, the ethical question will cease to be whether it is permissible to use gene editing to prevent disease. Instead, ethicists will be asking whether it is ethically permissible not to make interventions that would avoid human pain and suffering.

All this will take time — possibly as long as a decade, depending on how many scientists break the current norms and how well their patients do. But if the science works, expect the ethical norms to follow. Forty years ago, ethicists fretted about in vitro fertilization. Today, the practice seems ethically unproblematic or even attractive to the public in most countries.